The goal of these studies is to understand the unique biology of metastatic breast cancer cells, and to utilize this information to identify new drug targets for inhibition of metastatic progression. This study combines both discovery-based and hypothesis-driven approaches to investigate breast cancer metastasis. In the discovery phase of the project (Aims 1 and 2), I will focus heavily on the use of single-cell mRNA sequencing technologies to investigate cellular heterogeneity at early versus advanced stages of the metastatic process. Sequencing data will be used to generate global gene expression signatures that define metastasis-initiating cells, and transcriptomic changes that occur as these cells proliferate and advance to later stages of metastatic disease. Sequencing data will also be analyzed to identify druggable targets on metastasis-initiating cells, as well as targets that appear critical for transition of these cells into later stages of metastasis. The second phase of the project (Aim 3) will test the functional relevance of candidate drug targets in metastasis, and whether manipulation of potential targets can attenuate metastatic disease. These experiments will largely be performed using PDX models of human breast cancer, because they represent the most authentic pre-clinical models of human breast cancer to date. Furthermore, the species difference between the human cancer cells and the mouse host provides several key experimental advantages, which I have exploited in order to develop a robust new model system for high resolution analysis of metastatic disease. I strongly believe that these studies will lead to breakthroughs in the understanding of the basic biology of breast cancer metastasis, and also yield information that can be translated to the clinic in the short and long term.